1. Field of the Invention
This invention relates to a developing apparatus for use in an electrostatic recording apparatus such as an electrophotographic reproducing machine.
2. Description of the Prior Art
As is known in the art, a conventional toner developing apparatus has a construction such as shown in FIG. 1. The toner developing apparatus includes a box which stores therein a developer T such as one-component developer or two-component developer. A rotary sleeve 3 is placed inside the box 1 in the proximity of the circumferential surface of a photosensitive drum 2, and a plurality of permanent magnets 4 are positioned inside the rotary sleeve 3 with their poles directed in the radial direction of the rotary sleeve 3. The tip 5a of a scraper plate 5 extending in the tangential direction is brought into linear contact with the outer circumferential surface of the rotary sleeve 3 so that the developer on the circumferential surface is scraped off by the scraper plate 5 and frictionally charged by agitation blades 6.
Since high speed development has been required recently, the latest models of toner developing apparatuses increase the rotational speed of the rotary sleeve 3 and use a developer having a high electric insulating property, especially those having a spherical carrier. In such high speed developing apparatuses, the thickness of the developer deposited on the circumferential surface of the rotary sleeve 3 is as thin as from 0.5 to 1.5 mm, so if the rotary sleeve 3 has a smooth surface transfer of the developer is partially cut off, or a so-called "shortage" of developer occurs so that a part of the electrostatic latent image can not be developed. Accordingly, in developing apparatuses which use a thin developer layer, the circumferential surface of the rotary sleeve 3 is sometimes intentionally roughened to improve the transfer power between the circumferential surface and the developer. With this construction, however, the scraper plate 5 is likely to come into linear contact with the circumferential surface of the rotary sleeve 3 even if it is disposed in such a manner that it does not come into contact with the surface of the rotary sleeve 3 but scrapes off the developer therefrom. In consequence, the roughened surface on the rotary sleeve 3 is rapidly worn out by the tip 5a of the scraper plate and the developer can not be scraped off partially from the rotary sleeve 3. Needless to say, the scraper plate 5 itself is also worn. In this case, the rotary sleeve 3 for developing itself must be replaced.
As described above, in a dry developing apparatus generally including a plurality of fixed magnets disposed in an arc and a rotary sleeve of a non-magnetic material adapted to rotate around the outer circumference of the fixed magnets, the image receptor for forming an image, i.e. the photosensitive drum, is in most cases adapted to rotate clockwise for convenience in the arrangement of an image forming processing portion and for smooth transfer of the image, if the apparatus employs means for transferring a developer consisting of a carrier and a toner or a one-component developer integrating the carrier with the toner. On the other hand, the rotary sleeve for transferring the developer is rotated from below to above, or clockwise, in the developing apparatus in order to transfer a sufficient quantity of developer from the developer reservoir at high speed without scattering. Accordingly, in the developing process, the photosensitive drum and the rotary sleeve unavoidably move in opposite directions and the developer impinges against the surface of the photosensitive drum too strongly. This problem can be prevented by moving both the rotary sleeve and the photosensitive drum in the same direction in the developing portion. In order to rotate the rotary sleeve counter-clockwise and to transfer the developer to the developing portion sufficiently smoothly, however, fixed magnets must be continuously disposed inside the rotary sleeve. Since no gap exists in the magnetic portion in this case, it becomes extremely difficult to replace the developer. Because of this problem, the conventional system rotates the photosensitive drum and the rotary sleeve for developing in opposite directions and does not dispose the magnets at the upper scraping portion of the developer to facilitate the replacement of the developer. In developing with this system, a large strong magnet is used for the main developing magnet portion. In this case, magnetic iron powder of amorphous state is used as the carrier and the magnetic force generates the form of a high ear L.sub.1 via the magnetic iron powder as shown in FIG. 2. In this case, the gap between the surface A of the photosensitive drum and the rotary sleeve is from 4 to 5 mm and the developer does not scatter by the surface A of the photosensitive drum because sufficient magnetic attraction acts upon the carrier at the tip of the ear. However, the magnetic iron powder of amorphous state has the problem of low durability and the developer must be replaced frequently.
In recent years, improvements in the developer, especially the carrier, have made remarkable progress and a spherical carrier having an insulating agent such as resin coated on the outer circumference of the carrier has been developed. The spherical carrier has durability of about 5 times that of the conventional magnetic iron powder of amorphous state and is effective in reducing the number of times the developer must be replaced. Contrary to its high durability, however, the spherical carrier is somewhat difficult to transfer. This is because the surface of the carrier is coated with the insulating material and the carrier has low magnetic binding force and insufficient attraction. In addition, since the carrier is spherical, the contact surface between the carrier particles is small and the force of attachment to the rotary sleeve for developing and the transfer power are low. To obtain a satisfactory picture image, the gap between the surface of the developing cylinder and the surface of the photosensitive drum is most preferably from 0.5 to 15 mm. If the ear for development of the carrier is formed as high as the ear of the carrier of amorphous state especially in the developing portion where it comes into contact with the surface of the photosensitive drum, the carrier is thrown off against the attraction of the magnetic force by the turning force of the surface of the photosensitive drum and drops in large quantities.
Furthermore, there is some non-uniformity in the sensitivity of the photosensitive member on the surface of the photosensitive drum. This non-uniformity of the sensitivity generates non-uniformity of the charge potential of the photosensitive drum in the axial direction and results in non-uniformity of the image density. Two adjusting means have conventionally been employed to prevent this non-uniformity of the sensitivity of the photosensitive member from resulting in non-uniformity of the density. One of them moves a charging were so as to change the gap between the wire and the surface of the photosensitive drum and make the charge potential uniform. As shown in FIG. 3(a), for example, when the charge potential E of the photosensitive drum uniformly increases (or decreases) with the distance x from one end of the drum, the gap between the charging wire and the surface of the photosensitive drum is adjusted to increase (or decrease) with the distance x, thereby making the charge potential uniform and eliminating the non-uniformity of the density. However, it is structurally difficult to modify the charging wire in the form of a bent line, and adjustment is not possible for a photosensitive drum showing a charge potential as shown in FIGS. 3(b ) and 3(c). The other adjustment means adjusts the light distribution to the photosensitive drum. In the case of a photosensitive drum showing the characteristics shown in FIG. 3(b), for example, the amount of light at the ends of the photosensitive drum is increased compared with the amount of light at the center. Though this adjustment is effective for low density portions, it is completely ineffective for so-called "solid" portions which are not affected by the amount of light.